Strain clamp for cables and the like



April 28, 1959 R. L. LOCK ETAL STRAIN CLAMP FOR CABLES AND THE LIKE Filed Sept. 26, 1955 3 Sheets-Sheet 1 'INVENTORS ROBERT L. LOCK NILS L. NILSSON SAMUEL \l. SPURGEON BY 122m AT ORNEYS April 1959 R. L. LOCK ETAL STRAIN CLAMP FOR CABLES AND THE pm:

. s sheets-sheet 2 g a i INVENTORS R OB ERT L LO CK Nl L5 L. N l LSS N SAMUEL J. SPURGEON B-Y {gin TTORNEYS Filed Sept. 26, 1955 April 28, 1959 R, L. LocK EFAL STRAIN CLAMP FOR CABLES AND THE LIKE Filed Sept. 26, 1955 &N

3 Sheets-Sheet 3 INVENTORS ROBERT L LOCK NILS 1.. NILQSON SAMUEL J spunccow 8W TORNEYS w m A w United States Patent 2,884,476 STRAIN CLAMP FOR CABLES AND THE LH(E Application September 26, 1955, Serial No. 536,453

" 2 Claims. (Cl. 174-42) Our invention relates to strain clamps for cables and the like, and relates to such a device especially adapted for use in securing electrical conductors under those circumstances where they are to be dead-ended. 'Heretofore in the art to which our invention relates various attempts have been made to increase-the holding capacity of strain clamps. In some instances the surfaces of the cable receiving groove have been roughened or serrated. One of the principal ways of attempting to increase the holding power of such clamps heretofore has been 'to make the cable groove and keeper with complementan'ly curved or wavy portions thereon. It has been considered that the wavy configuration of the keeper and groove, when the keeper is pulled down tightly onto the cable,-increases the holding power of the clamp. Howeve-r, our investigation of the problem has proven that merely curving the groove and keeper will not increase materially the holding ability of such a clamp. Our investigation leads us to the conclusion that merely curving or'waving the groove and keeper is not fully effective because a cable, especially a multiple stranded one, when put under compressive stress, immediately exhibits the phenomenon of cold flow. The cold flowing continues until the cable reaches a point of equilibrium insofar as internal stresses thereon are concerned which may be imposed by clamping pressure and axial load. For instance, in a multiple stranded cable the strands initially contact each other on line or point contacts at places where the strands cross or contact each other. When compressed the metal adjacent these contacts flows, thereby dccreasing the efifective diameter of the cable by an appreciable number of thousandth-s of an inch. In prior clamps, even though provided with sinuous cable receiving grooves and the like, cables continue to slip axially due to the foregoing condition. Further, prior clamps of this'general type fail to utilize the strength of all the strands of a cable by failing to distribute the tensile load among all strands of the cable. This failure is due to the fact that individual strands are initially overloaded and remain overloaded, there being no way in such prior clamps evenly to distribute the tension on each strand. Suffice it to say that our experience with previous strain clamps has shown:

a) That no such clamp with which we are familiar, which is of economical size and weight, will compensate for cold flow of the metal in the cable; and,

(b) No such clamp will so engage the conductor that all parts or strands carry proportionate parts of the tensile stress on the cable.

In View of the foregoing an object of our invention is to provide a strain clamp for cables in which there is means frictionally to secure the keeper thereof to the cable, whereby upon reduction of the diameter of the cable in the clamping portion of the clamp the keeper moves forwardly, together with means effective upon forward movement of the keeper to move it still closer towardthe body of the clamp, thus obtaining increased holding power and more even distribution of the tension 2,884,476 Patented Apr. 28, 1959 stress throughout the cable, assuring even distribution of strain therein.

Another object is to provide a strain clamp for cables in which the body of the clamp has a cable receiving groove therein with a keeper adapted to be drawn down onto a cable lying in the groove, there being curvedsections of the keeper and groove adjacent the rear end of the keeper which coact on their rearward surfaces frictionally to secure the keeper to the cable, the cable being substantially unclamped on the forward surfaces of said curved sections, together with clamping bolts or the like which pivotally connect the keeper to the body, whereby, upon forward slipping of the cable in the groove the keeper moves forwardly and due to such movement is drawn still further toward the clamp body, onto the cable.

Our invention contemplates a clamp of the character described in which the keeper is driven forwardly from its rear end by the snubbing action of the cable on the keeper, and in which a forward clamping section of the keeper and groove exert a clamping force upon the cable somewhat in proportion to the axial tensional stress on the cable, thus compensating for cold flow or distortion of the cable to assure continuous, even and high pressure clamping of the cable adjacent the forward end of the clamp.

A still further and more specific object is to provide a clamp of the character designated in which the rear sur faces of the coacting curved sections of the keeper and cable groove are of such relative configuration that upon forward movement of the keeper such surfaces come closer together, thus insuring continued snubbing action between the cable and keepers adjacent the rear end ofthe keeper. 7

A strain clamp illustrating features of our invention is shown in the accompanying drawings forming a part of this application, in which:

Fig. 1 is a plan view thereof showing a cable in place in the clamp with the clamp in the initial assembled position;

Fig. 2 is a side elevational view of the clamp in the position shown in Fig. 1, certain of the parts being broken away and in section;

Fig. 3 is an exploded view of the clamp with the cable omitted, certain parts being broken away and in section;-

parts assume after the cable has reached a point of equilibrium of strain therein;

Fig. 5 is a wholly diagrammatic view illustrating the relative changes in position of the parts upon shifting of the keeper;

Fig. 6 is a wholly diagrammatic view illustrating the forces involved and the directions and components there-' of;

Fig. 7 is a cross sectional view of a section of stranded conductor showing the configuration of the various strands prior to cold flowing of the same; and,

Fig. 8 is a view similar to Fig. 7 and showing the configuration of the strands after the metal has been subjected to compressive forces for a length of time.

Referring now to the drawings for a better understanding of our invention, our improved clamp comprises essentially a body indicated generally by the numeral 10and a keeper indicated generally by the numeral 11. The body is provided with a longitudinally extending conductor rethe cable C by means of Ubolts 16 and nuts 17 therefor on the threaded ends of the U-bolts. The keeper may be provided with saddles 18 to receive the bolts. The body is provided with bolt holes 19 which receive the bolts 16 rather loosely and which are elongated longitudinally of the clamp body as clearly shown in the drawings.

By reference particularly to Figs. 2 and 4 it will be connection of the bolts, this forward motion will result in the keeper moving downwardly a distance indicated at 39, thus moving toward the cable and clamping it more securely in the areas indicated by the arrows 28 and 32. It will be noted that the relative curvatures and configurations of the curved portions 26 of the keeper and curve 23 of the groove are such that as the keeper moves forward the rear of the keeper moves slightly toward the cable. However, it will be apparent that the main clamping action and hence the main holding power of the clamp is derived from the elongated, relatively straight forward area indicated by the arrow 32.

section 26 which is generally complementary to the curved section 23 of the groove. However, it will be noted that the relative configuration of the curved groove section 23 and the downwardly curved section 26 of the keeper is such that the cable is clamped substantially only in that area indicated by the cross dotted lines 27 in Fig. 5. This clamping area 27 extends substantially the distance indicated by the arrow 28 in Fig. 6. Thus, the rearmost curved surfaces of the curved sections 23 and 26 serve to clamp the cable very firmly and to exert a very high pressure thereon in response to drawing downward on the adjacent U-bolt 16. However, the forward surfaces of these curved portions are of such relative configuration as to leave an unclamped section of the cable which is indicated in Figs. 2 and 4 by the space 29 and in Fig. 6 by the arrow 31. Therefore, it will be apparent that the high unit pressure of contact exerted by the rearward curved end portion 26 of the keeper, on the cable C, serves frictionally to secure the cable C to the keeper. At the same time, due to the substantially unclamped cable portion 31 on the forward surfaces of the curved portions 23 and 26, the keeper is free to move forwardly in response to pull on the cable. The clamping portion proper of the cable extends from the point 21 forwardly to the end of the keeper and is indicated in Fig. 6 by the arrow 32.

With the foregoing construction in mind the operation of our improved clamp and its several advantages may now be fully explained and understood. With the cable C held in place (by means not shown) in the clamp, the keeper is drawn down onto the cable by means of the U-bolts 16 and their nuts 17. Initially, the bolts may be in the position shown in Figs. 1 and 2, that is, lying generally at right angles to the groove 12, as the device is viewed in side elevation. When thus assembled and when tension is applied to the cable '0 in the direction of arrow 33, it will be seen that the metal in the conductor, particularly if it be a stranded conductor, commences to cold flow, especially where the strands cross each other. As this cold flow progresses the effective diameter D decreases from that shown in Fig. 7 to that shown in Fig. 8. Due to the provision of the high unit pressure clamping section 27 and due to the provision of the elongated holes 19, the keeper is free to shift forwardly from the position of the bolts shown in Figs. 1 and 2 to the position thereof indicated in Fig. 4 and in full lines, Fig. 5. In order to facilitate the pivotal action of the bolts we preferably round the lower ends of the bosses in which the holes 19 are located, as indicated by numeral 34.

, By referring particularly to Figs. 5 and 6 it will be readily appreciated that we maintain at all times a clearance or non-compressive action between the forward surfaces of the curve 26 of the keeper and curve 23 of the groove. Therefore, when the keeper moves from the initial position indicated by the vertical line 36 in Fig. 5 to the full line position of the bolts indicated by the numeral 37, the keeper will have moved forwardly a distance indicated by the arrow 38. Due to the pivotal From the foregoing it will be apparent that we have devised an improved strain clamp for cables. It is to be especially noted that the components of force as shown in Fig. 6, at the rear end of the keeper, are such that the keeper positively is frictionally secured to the cable. Therefore, the keeper 11 is driven forwardly by being pushed from its rear end due to this frictional engage-v ment of the cable at that point, upon axial movement of that part of the cable. This feature, together with the maintaining an unclamped portion forwardly of the rounded portions 23 and 26, together with means so connecting the keeper to the body that it moves toward thebody upon axial movement, afiiord a clamp which is greatly improved over existing clamps of this general type. It is to be noted that we employ the curve in the keeper and groove for an entirely different purpose from that to which similar curves have been put in prior clamps of this type. Thus, in our improved clamp, we do not attempt to increase the frictional hold on the clamp forholding purposes by the provision of the curved surfaces; on the other hand, we employ this solely for the purpose of snubbing forwardly the keeper when the cable moves and we employ additional means in the form of U-bolts and the like to translate a portion of thisforward motion into an increased clamping force.

In actual tests we have found that our clamp holds a cable without breaking substantially to its theoretical ultimate breaking strength. It will be noted that by maintaining an increased and constant pressure upon a relatively long length of the cable indicated by the arrow 32, each strand of a stranded cable and, each cross sec-. tional portion of an unstranded cable, is caused to carry, a proportionate part of the tensional load. Our clamp thus increases the frictional engagement among the strands, resulting in increasing the tensile strength of the.

What we claim is: 1 i 1. In a cable clamp having a forward end adjacent atensioned portion of the cable to be secured and a rear end opposite said forward end adjacent a slack portionof the cable, a body having a continuous longitudinally extending groove adapted to receive the cable, said cable groove having a generally straight section adjacent the.

forward end of the body and a reversely curved concave section adjacent the rear end of said body, a keeper having a generally straight cable clamping section adjacent the forward end thereof coacting with the straight section of the groove to clamp the cable therebetween, a reversely curved convex projection adjacent the rear end of the keeper, said projection being generally complemem tary in shape to the shape of the concave groove section, the rear portion of said reversely curved projection and the rear portion of said reversely curved section of the groove being constructed and arranged to clamp the cable therebetween, thereby to snub the cable to the rear end of the keeper, the forward portion of the concave section of the groove and the forward portion of the convex projection being relatively arranged to exert less pressure on that part of the cable lying therebetween than is exerted on the cable by the coaction of said rear portion of said projection and said rear portion of the reversely curved groove section, and clamping means effective to draw the cable clamping sections of the keeper onto the cable upon forward movement of the keeper responsive to tensioning of the cable.

2. The combination with a dead. end clamp for cables of the kind having a body with a cable groove running substantially the length thereof and comprising a straight section at one end and a reversely curved concave section at the other end, together with a keeper having a straight section cooperating with the straight section of the groove to clamp the cable therebetween, of a reversely curved convex projection generally complementary to the shape of the concave groove section and located adjacent the opposite end of the keeper from the straight section, the curved portions of the keeper and groove farthest removed from the straight section of the keeper .6 and groove being arranged to clamp the cable lying therebetween and thereby effectively to snub the keeper to the cable, the curved portions of the keeper and groove adjacent the straight section thereof being shaped to exert relatively less clamping action on the cable therebetween than the other curved section of the keeper and groove, and'means effective to draw the cable clamping portion of the keeper onto the cable and to hold them tightly clamped upon shifting of the keeper relative to the clamp body when the cable is tensioned.

References Cited in the file of this patent UNITED STATES PATENTS 1,064,920 Lovejoy June 17, 1913 1,073,658 Bolus Sept. 23, 1913 1,170,725 Austin Feb. 8, 1916 1,584,848 Clowes May 18, 1926 1,667,345 Cummins Apr. 24, 1928 1,873,798 Varney Aug. 23, 1932 2,453,432 Hackathorn Nov. 9, 1948 

